This invention is based on patent application No. 20005-12618 Pat. filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The invention relates to a method of forming a silicon-contained crystal thin film which can be used, e.g., in substrates for a solar battery, a liquid crystal display and a semiconductor device.
2. Description of the Background Art
In recent years, attention has been focused on a hydrogen ion injection peeling method as a method for forming a silicon-contained crystal thin film on a support substrate. The hydrogen ion injection peeling method is utilized for producing an SOI (Silicon On Insulator) substrate, a solar battery substrate and others.
An example of a method of forming an SOI substrate by the hydrogen ion injection peeling method will now be described with reference to FIG. 9.
First, a thermal oxide film (SiO2 film) 911 is formed on an Si crystal substrate (see a step (B) in FIG. 9).
Then, hydrogen ions are injected into the Si crystal substrate 91 from the side of the thermal oxide film (see a step (C) in FIG. 9). By an ion acceleration voltage, an injection depth of hydrogen ions is controlled. In a later step, the crystal substrate 91 is separated (divided) in a position 913 where hydrogen ions are injected as described above. For this purpose, the hydrogen ions are injected at an injection density of about 5xc3x971016 ions/cm2 or more.
Then, a support substrate 92 is laid on the ion-injected surface (thermal oxide film) of Si crystal substrate 91, and they are heated so that many voids or fine holes 914 are formed in the ion-implanted position 913 of the Si crystal substrate 91 (see a step (D) in FIG. 9). The hydrogen injected into the Si crystal substrate 91 is gasified by the heat so that the voids 914 are formed. The heating for forming the voids 914 is performed for several minutes at a temperature of about 350xc2x0 C.-600xc2x0 C. The neighboring voids are connected together. As a result of this connection and others, a weak or fragile portion having a layer-like form is formed in the void-formed portion of the Si crystal substrate 91. Since the ion-injected surface (thermal oxide film) of the Si crystal substrate 91 is subjected to a pressure applied by support substrate 92, this suppresses such a situation that the surface portion of the substrate 91 is partially peeled off due to the pressure by the hydrogen gas. Peeling which produces crater-like portions of several micrometers or less in diameter would occur in the surface portion of the crystal substrate 91 unless a pressure is applied to the ion-injected surface of the Si crystal substrate 91 by the foregoing manner or another appropriate manner.
Then, the Si crystal substrate 91 and the support substrate 92 are heated and adhered together at a high temperature of about 1000xc2x0 C. or more.
Thereafter, the Si crystal substrate 91 is divided along the voids 914 (see a step (E) in FIG. 9). Thereby, an Si crystal thin film 9121, which was a portion of the crystal substrate 91, is left on the SiO2 film 911. In this manner, the SOI substrate, i.e., the support substrate 92, on which the Si crystal thin film 9121 and the SiO2 film (insulating film) 911 are layered, is formed. An Si crystal substrate portion 9122 other than the above can be used again in the next process of forming the SOI substrate.
However, the following problems arise in the above method of forming the silicon-contained crystal thin film on the support substrate, e.g., by implanting ions into the silicon-contained crystal substrate to form the voids.
One of the problems is that the foregoing method cannot efficiently form a relatively thick silicon-contained crystal thin film of tens of micrometers in thickness on the support substrate without difficulty. This will be described below in greater detail.
In the case where the substrate provided with the Si crystal thin film is to be utilized for manufacturing a solar battery, the Si crystal thin film must have a thickness of 7 pm or more, and more preferably, about 10 xcexcm for achieving a high photoelectric conversion.
The thickness of the Si crystal thin film formed on the support substrate corresponds to the ion implantation depth in the hydrogen ion implanting processing, and can be increased by increasing the ion implantation depth. By increasing the ion acceleration voltage in the ion implantation process, the ion implantation depth can be increased within a limited extent. For implanting hydrogen ions to a position at a depth of, e.g., 10 xcexcm from the surface (ion-injected surface) of the crystal substrate, the ion implantation is performed with an acceleration voltage of about 700 keV. However, for achieving the implantation depth of about 10 xcexcm or more, a large-scale ion implanting device is required so that the ion implanting process requires a high cost. For suppressing excessive increase in temperature of the crystal substrate subjected to the ion implantation, it is necessary to suppress an ion beam current, and therefore it is impossible to perform efficiently the ion implantation achieving a required density.
According to the above manner, it is difficult to produce a relatively thick silicon-contained crystal thin film with high efficiency and low cost.
Another problem also arises. If particles are present on the ion-injected surface of the Si-contained crystal substrate, peeling of minute portions and/or cracks occur in the surface layer of the crystal substrate when heating the substrate for forming the voids. This will be further described below with reference to FIG. 10.
As can be seen in a step (A) shown in FIG. 10, a particle 93 is present on an ion-injected surface 915 of crystal substrate 91, and is interposed between the support substrate 92 and the crystal substrate 91. In this case, the ion-injected surface 915 has a portion, which is not pressed by the support substrate 92. When the heating for forming the voids is performed in the above state, peeling of minute portions and/or cracks may occur in and around the portion, which is not pressed by the support substrate 92, of the surface layer of crystal substrate 91, as can be seen in a step (B) shown in FIG. 10. Such minute peeled portions and cracks, which are present during the formation of voids, will cause a defect in the final product, i.e., the substrate provided with the crystal thin film. This lowers the productivity of the silicon-contained crystal thin films, and increases the manufacturing cost and selling price of the silicon-contained crystal thin film.
For example, it can be envisaged that the foregoing problem due to particles can be suppressed by executing the respective steps in a clean room of a high cleanliness. However, the clean room of a high cleanliness, which is required for overcoming the above problem due to particles, requires high construction, maintenance and operation costs, and thus increases the manufacturing cost of the substrate with the silicon-contained crystal thin film.
Accordingly, an object of the invention is to provide a method of forming a silicon-contained crystal thin film, which allows formation of the silicon-contained crystal thin film with high productivity and low cost.
More specifically, an object of the invention is to provide a method of forming a crystal thin film containing silicon, and particularly a method of forming a silicon-contained crystal thin film, which allows formation of the silicon-contained crystal thin film of a relatively large thickness with high efficiency.
Another object of the invention is to provide a method of forming a crystal thin film containing silicon on a support substrate, and particularly a method of forming the silicon-contained crystal thin film on the support substrate, which allows formation of the silicon-contained crystal thin film of a relatively large thickness with high efficiency.
Still another object of the invention is to provide a method of forming a crystal thin film containing silicon, e.g., by implanting ions into the crystal substrate containing silicon and forming voids, and particularly a method of forming the silicon-contained crystal thin film, which can suppress peeling of minute portions and generation of cracks in the crystal substrate during the process of forming the voids, even if particles are present on an ion-injected surface of the silicon-contained crystal substrate.
A further object of the invention is to provide a method of forming a crystal thin film containing silicon on a support substrate, e.g., by implanting ions into the crystal substrate containing silicon and forming voids, and particularly a method of forming the silicon-contained crystal thin film on the support substrate, which can suppress peeling of minute portions and generation of cracks in the crystal substrate during the process of forming the voids, even if particles are present on an ion-injected surface of the silicon-contained crystal substrate.
Basically, the invention provides first and second types of methods of forming a silicon-contained crystal thin film, which will be described below.
(1) Method of Forming a Silicon-Contained Crystal Thin Film of First Type
A method of forming a silicon-contained crystal thin film of the first type includes:
an ion implanting step of implanting hydrogen ions or helium ions into a silicon-contained crystal substrate containing silicon at 10% or more by weight;
a void forming step of forming a void in an ion-implanted position of the crystal substrate by heating the crystal substrate subjected to the ion implantation;
an epitaxial growth step of immersing the ion-implanted crystal substrate in a melted metal liquid containing silicon, and cooling the liquid to cause epitaxial growth to form a monocrystalline or polycrystalline thin film primarily made of silicon on an ion-injected surface of the substrate; and
a dividing step of dividing the crystal substrate provided with the crystal thin film in the void-formed position, wherein
the heating of the crystal substrate performed for the void formation in the void forming step is performed by immersing at least the ion-injected surface of the crystal substrate in the melted metal liquid used in the epitaxial growth step.
(2) Method of Forming a Silicon-Contained Crystal Thin Film of Second Type
A method of forming a silicon-contained crystal thin film of the second type includes:
an ion implanting step of implanting hydrogen ions or helium ions into a silicon-contained crystal substrate primarily made of silicon;
a film forming step of forming a predetermined film on an ion-injected surface of the silicon-contained crystal substrate;
a void forming step of forming a void in an ion-implanted position of the crystal substrate by heating the crystal substrate provided with the film; and
a dividing step of dividing the crystal substrate in the void-formed position, wherein
the film forming step is performed to provide a thickness of 2 xcexcm or more from a surface of the film to the on-implanted position.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.